Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing a new design.

The broken lines showing the display screen or portion thereof and the remainder of the graphical user interface are included for the purpose of illustrating environment and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 